This invention relates to a non-destructive testing method for determining the texture of materials using ultrasonics.
The word xe2x80x9ctexturexe2x80x9d designates direction-dependent properties of materials. One direction-dependent property of particular interest is elastic anisotropy in polycrystalline materials that results from the non-random distribution of the crystallographic orientations of single grains. Crystallographic texture is described by an orientation distribution function (ODF). Information on ODF is usually obtained from pole figure X-ray diffraction and typically consists of thousands of diffraction data points.
Conventional texture analysis of materials normally involves destructive testing. A small sample is cut off from a material and tested in a laboratory. In some cases, especially in production control, it is not necessary to determine the xe2x80x9cwholexe2x80x9d texture. In this case, it is possible to use a low-resolution texture analysis method which relies upon a strict correlation between some material physical properties such as, for example, elastic or magnetic properties, and crystallographic texture. By restricting the texture analysis to a low-resolution technique, it is possible to perform texture analysis in a non-destructive way that offers the possibility of on-line qualitv control inspection.
Three different techniques for low-resolution texture analysis are known. A first technique consists of taking X-ray measurements of a material under test. A device, called an xe2x80x9cOn-Line Texture Analyzerxe2x80x9d, designed and used for this purpose, irradiates a sample with an incident beam containing a continuous spectrum of wavelengths such as, for example, the X-ray bremsstrahlung spectrum. Characteristic pole-intensities of the sample are measured by energy-dispersive detectors detecting the X-ray bremsstrahlung spectrum transmitted through the sample material. However, this technique is limited to relatively small thicknesses of material. This limitation is due to strong X-ray attenuation and dispersion inside the polycrystalline material, and strict requirements for positioning of the X-ray source and detectors with respect to the texture of the material sample.
A second technique consists of electromagnetic Barkhausen noise and dynamic magnetostriction measurements. However, this technique is limited to materials having strong magnetic anisotropy.
A third technique is based on the measurement of a material""s vibrational properties, such as an ultrasound velocity, which are known to be correlated with the material texture. Ultrasound velocity measurements have advantages over the first two techniques in that samples to be tested are not limited in thickness, and materials without strong magnetic anisotropy may be analyzed.
A prior art ultrasonic method for low-resolution texture analysis of single-phase polycrystalline materials such as, for example, low-alloyed aluminum having a cubic structure and orthorhombic texture is depicted in FIG. 1. This technique employs a pulse-echo method to determine three ultrasound absolute propagation velocities (with respect to the specimen coordinate system) propagating in the rolling, transverse, and normal directions.
A single, short-duration, high-frequency ultrasound pulse 10, generated by an ultrasonic transducer 12, advances into a specimen 14 which has flat, parallel surfaces. Multiple reflections of ultrasound inside the specimen 14 results. A series of consecutive echos 16 (see FIG. 1A) with gradually decreasing amplitudes are generated. The echos 16 are received by the transducer 12 for calculation of the propagation velocity. The propagation velocity may be calculated using measurements of ultrasound round-trip path length and ultrasound round-trip time-of-flight. The round-trip path length may be determined as a doubled specimen thickness (L in FIG. 1) precisely measured in the direction of ultrasound propagation. The ultrasound time-of-flight may be measured as a time interval or period 17 between the leading edges of two consecutive echos 16. The absolute propagation velocities, calculated as a ratio of round-trip path length to time-of-flight, are usually used to determine the elastic constants (fourth-order expansion coefficients of the elasticity matrix) which characterize the texture of the specimen 14.
However, the accuracy of time-of-flight measurements may vary substantially depending on a number of factors such as: ultrasound pulse frequency spectrum; pulse rise time, length and shape; transducer-to-specimen positioning and coupling; and frequency band, resolution and accuracy of the electronic receiving system. The accuracy of the time-of-flight measurements is especially critical for materials having a low elastic anisotropy factor such as, for example, low-alloyed aluminum. In order to obtain acceptable measurement accuracy, out-of-line laboratory measurements may be required. The prior art ultrasonic method for texture characterization may not be suitable for on-line texture analysis.
A method of on-line ultrasonic texture characterization of a sputtering target is provided. Texture characterization may be accomplished through analysis of an ultrasonic backscattering signal amplitude distribution. A broad-band, focused ultrasonic transducer generates a megacycle center frequency ultrasonic pulse having a wavelength in the range of the average grain size (in the direction of ultrasound propagation) of a sputtering target specimen. The ultrasonic pulse is introduced into the specimen at an incident angle normal to the surface of the specimen. Due to interaction of the ultrasonic pulse with the texture of the specimen, backscattering echoes are generated in a portion of the specimen located within the transducer focal zone. The backscatter region extends at least one grain layer beneath the specimen surface to a depth of several grain layers in thickness. The backscattering echoes propagate back to the transducer where the echoes are converted by the transducer into an electrical signal which is processed by a broad-band acquisition system. A maximum amplitude value of the backscattering signal is extracted from the processed data and stored in a memory of the acquisition system for future data analysis. Data analysis is performed using data graphical representation in the form of a histogram of xe2x80x9coccurrences versus amplitudexe2x80x9d where the amplitude is plotted along the x axis while the occurrences (counts for certain amplitude values) are plotted along the y axis. The histogram is compared with histograms for reference standards having known preferred crystallographic and grain orientation, grain size, and chemical composition.
Therefore, it is an object of the invention to provide a method of ultrasonic on-line texture characterization.
It is a further object of the invention to provide a method of on-line texture characterization including the step of generating an ultrasonic pulse with a wavelength in the range of the average grain size of a specimen material in the direction of ultrasound propagation.
It is yet another object of the invention to provide a method of ultrasonic on-line texture characterization including the step of detecting an ultrasonic backscattering signal generated by interaction of an initial ultrasonic pulse with specimen material texture.
A still further object of the invention is to provide a method of ultrasonic on-line texture characterization including the step of plotting the backscattering signal amplitude in the form of a histogram of xe2x80x9coccurrences versus amplitudexe2x80x9d, and comparing the histogram with similar histograms for materials having known preferred crystallographic and grain orientation, grain size, and chemical composition.
Other objects of the invention will be apparent from the following description the accompanying drawings and the appended claims.